Just to note, I am pre-Champion so access to the Super Rod can't be done

Here's some of the ones I'm aware of, since you're playing D/P and not Platinum.

Spoiler:

Rampardos - Victory Road, used by Ace Trainer Omer.

Machamp - Victory Road, used by Black Belt Miles.

Blissey - Victory Road, used by Ace Trainer Mariah.

Clefable - Victory Road, used by Ace Trainer Sydney.

Gabite - Victory Road, used by Dragon Tamer Clinton.

Riolu - Oreburgh Gate basement, used by Veteran Grant.

Mantine - Route 223, used by Swimmer Oscar.

Lumineon - Route 223, used by Swimmer Miranda.

Tentacruel - Route 223, used by Swimmer Ricardo and Swimmer Wesley. You can also occasionally find wild ones on various water areas when surfing. Pretty much the same areas where wild Tentacool can be found.

So pretty much you can find most of them in some upcoming trainer battles, except for Riolu who you'll need to backtrack a bit to since Oreburgh Gate was one of the early-game locations. Just make sure you have Surf though.